The present invention relates to a television synchronizing signal reproducing apparatus which receives a television composite video signal and reproduces synchronizing signals such as a composite synchronizing signal including horizontal and vertical synchronizing signals, a composite blanking signal, a burst flag signal, burst signal, etc.
During transmission or processing of a television video signal, both of the synchronizing signals and an associated burst signal are subject to distortion and their levels are subject to variations due to either the transmission line or a processing circuit. Further, the positional relationship between the horizontal synchronizing signal and the burst signal is frequently altered during transmission or processing. The processing circuits may include a mixing circuit, a circuit for generating special effects, and the like.
The synchronization signals contained in a television video signal should be maintained at their predetermined standards, because they are used as references for the proper operation of a variety of television equipment such as a television monitor. In order to eliminate distortion, a television synchronizing signal reproducing apparatus is generally used to "reproduce" synchronizing signals, such signals being substituted for the original synchronizing signals contained in a television composite video signal which has been distorted during its transmission through the transmission line or the processing circuit.
In one prior art television synchronizing signal reproducing apparatus, a first circuit receiving a burst signal extracted from an input television video signal reproduces a subcarrier signal. A second circuit receiving horizontal and vertical synchronizing signals extracted from the input television video signal reproduces synchronizing signals such as a composite synchronizing signal, a composite blanking signal, a burst flag signal and horizontal and vertical driving signals. In this apparatus, the reproduced burst signal is "framed" by extracting the reproduced subcarrier in response to this burst flag signal. According to the prior art television synchronizing signal reproducing apparatus, the two circuits, i.e., the circuit for producing the subcarrier and the circuit for producing the synchronizing signal, including the burst flag signal operate independently of each other. Therefore, when the position-relationship between the synchronizing signals, particularly the horizontal synchronizing signal, and the burst signal contained in the input television video signal is changed, or when the horizontal synchronizing signal contained in the input television composite video signal is distorted, the leading edge and the trailing edge of the burst flag signal are not coincident with the zero crossing point of the reproduced subcarrier. As a result, the reproduced burst signal does not begin with a zero crossing point. In other words, the reproduced burst signal contains the distortion of the waveform and the phase error.
Generally, it is recommended that the beginning and the termination of the burst signal are coincident with the zero crossing point of the subcarrier signal, and that the period from the leading edge of the horizontal synchronizing signal to the beginning of the burst signal is kept to be an integer multiple of the period of one cycle of the subcarrier signal. However, the prior art television synchronizing signal reproducing apparatus described above does not satisfy these requirements.